1. Field of the Invention
This invention relates to a spread spectrum communication apparatus and a signal intensity detection apparatus. More particularly, it relates to a spread spectrum communication apparatus and a signal intensity detection apparatus designed to find the signal intensity at a base station.
2. Background of the Invention
In a mobile communication system, such as a so-called cellular system, it is crucial for a mobile terminal to measure the signal power intensity at the base station since such measured signal power intensity serves as a measure of stability of a call to be provided to the user. In a currently serviced analog cellular system, the signal intensity of the frequency channel presently received is measured and displayed in several stages.
A mobile communication system employing a code division multiple access (CDMA) system is a system characterized by the fact that the same frequency channel is shared by a plurality of base stations, and by the fact that, even for a sole base station, a plurality of code channels are multiplexed on the same frequency channel.
FIG. 1 schematically shows the usual cellular communication system. In FIG. 1, there is shown an example in which communication is had between a sole mobile terminal 46 and a plurality of, herein three, base stations 41, 42 and 43.
In an analog modulation cellular system or a time division multiple access (TDMA) cellular system, different frequency channels are employed for transmission by respective base stations. In the CDMA cellular system, each base station shares the same frequency channel with the aid of the spread spectrum technique. The spread spectrum communication system exploits pseudo random number series, known as a spread spectrum code or spread code, or a so-called PN code. However, by employing different random number series or temporally shifted random number series in different base stations, signals of a plurality of base stations may be multiplexed on the same frequency. It is necessary for a mobile terminal to be moved during a call from a service area of a base station to which it is initially connected to be connected to a different base station. This is termed hand-off. A hard hand-off, in which a mobile terminal has its network connected to one base station, is used with the analog modulation cellular system or a TDMA cellular system, while a soft hand-off, in which the mobile station has its network connected to a plurality of base stations simultaneously, is used with the CDMA cellular system.
In addition, with the CDMA cellular system, a plurality of channels on which signal transmission is made from respective base stations are multiplexed using the same frequency channel. With a forward link, that is, a link from a base station towards a mobile station, of the CDMA cellular system, there are provided four different code channels, namely, a pilot channel, a sync channel, a paging channel and a traffic channel. The pilot channel is a channel over which data is not transmitted and the above-mentioned PN code is repeatedly transmitted. The pilot channel is employed for synchronization capturing and maintenance by the mobile terminal and for clock generation. The sync channel is employed for matching the time information and long-term PN code between the base station and the mobile station. The paging channel is employed for transmission of the information required for hand-off, the information for calling the terminal on termination and the allocation information of the traffic channel. The traffic channel is employed for transmitting the speech information during a call.
FIG. 2 schematically shows the construction of a transmitting section of a base station.
With the CDMA cellular system, the above-mentioned respective channels are multiplexed by changing the spread codes to be multiplied by data and transmission is made using the same frequency. With the present system, the PN code is multiplied by a Walsh code to give a code used as a spread code and respective channels are generated by changing the Walsh code. With the pilot channel, the Walsh code is always zero such that the PN code is transmitted directly. Thus the detection of the PN code transmitted by the base station means checking for timing of the spread code of the pilot channel. It is possible for the mobile terminal to select the spread code generated by the PN code generator in order to demodulate data of the desired code channel. The pilot channel, however, is not a channel designed to take out data.
Referring to FIG. 2, the pseudo random numbers or the PN code from a PN code generator 51 are transmitted to multipliers 56, 57, 58 and 59, which are fed with the Walsh code from Walsh code generators 52, 53, 54 and 55, respectively. The Walsh code generator 52 transmits the Walsh code, which is perpetually zero (Walsh code 0) for the pilot channel, such that the PN code from the PN code generator 51 is directly routed via the multiplier 56 to a channel addition unit 60 as a pilot channel signal. The Walsh code generators 53, 54 routes pre-set Walsh codes, such as a Walsh code 32 and a Walsh code 1, to the multipliers 57, 58, respectively. Outputs of these multipliers 57, 58 are routed to multipliers 61, 62 for multiplication by sync channel data and paging channel data, respectively. The resulting multiplied outputs are routed to the channel addition unit 60. The Walsh code generator 55 routes the Walsh code other than the Walsh codes from the Walsh code generators 53, 54, that is, the Walsh code n where n is different from 0, 1 or 32, to a multiplier 59, an output of which is fed to a multiplexor 63 for multiplication by data from the traffic channel. The resulting multiplied data is routed to the channel addition unit 60. The channel addition unit sums the data from the respective channels and routes the sum data to a modulator of a base station.
FIG. 3 schematically shows the construction of a receiver of a mobile terminal of the CDMA cellular system.
Referring to FIG. 3, signals received over an antenna 70 is frequency-converted by a receiver 71 before entering an AGC circuit 80. An output of the AGC circuit 80 is routed to a base station spread code detection unit 79 termed a searcher and to a plurality of, herein three, demodulators 75, 76 and 77. The spread code detection unit 79 measures the signal intensity and the time points of the spread code contained in the pilot channel transmitted by the base station. The demodulators 75 to 77 separately demodulate signals with different delays generated by different paths, namely multipasses, or signals transmitted from different base stations for a soft hand-off. The demodulation operation by the demodulator 75 to 77 is executed on the basis of an output of the spread code detection unit 79. Outputs of these demodulators 75 to 77 are routed to a combiner unit 78 which combines respective demodulated outputs to output a combined demodulated signal having a high S/N ratio. The AGC circuit 80 is made up of an AGC amplifier 72, a detector 73 and an amplifier 74 for amplifying an output of the detector 73 for generating a control signal for the AGC amplifier 72. For providing a constant power of the amplifier 72, a control signal is fed back to the amplifier 72 via the detector 73 and the amplifier 74.
As discussed above, it is desirable to detect the reception signal intensity of the received signal as a measure of the state of call stability. Thus it may be envisaged to utilize an output of the detector 73 in the AGC circuit 80 of FIG. 3 converted into, for example, electric power, as the information representing the reception intensity.
FIG. 4 shows the contents of the received power for a frequency channel received by a mobile terminal. With the analog modulation cellular system or a TDMA cellular system, it is only the specified base station and moreover its specified channel, for example, a pilot channel of the base station 41 of FIG. 1, that the mobile terminal receives at the same frequency. Thus the signal intensity of the specified channel of the specified base station can be known by measuring the total received power. However, with the CDMA cellular system, the signals transmitted by a plurality of base stations are multiplexed on the same frequency.
That is, the powers E.sub.1 of the base station 41, E.sub.2 of the base station 42 and the power E.sub.x of other base stations are contained in the total received power E.sub.t on the same frequency channel, as shown in FIG. 4. In addition, the transmitted power of, for example, the base station 41, is made up of the powers of the pilot channel, sync channel, paging channel (comprising up to seven channels) and the traffic channel (i channels). The number of the traffic channels employed i is changed with the number of the terminals connected to the base stations. For measuring the signal intensity of a specified base station with the CDMA cellular system, it is necessary to measure the power of the pilot channel of the specified base station. However, since the ratio of the power of the pilot channel to the total received power is not constant, the intensity of the received signal cannot be measured sufficiently accurately with such a method of measuring the total received power.
The following method may also be employed for estimating the signal intensity from the base station.
Data transmitted from the base station is coded by convolutional coding. At a mobile terminal, the coded data is corrected for errors using Viterbi decoding. The signal intensity may be estimated by finding the error rate from the decoding process and by converting the error rate into the signal intensity. However, this method is in need of a considerable data volume in order to maintain estimation accuracy, moreover it has to demodulate and decode signals of the base station and hence is not suitable if the mobile terminal is in a stand-by state. For these reasons, more reliable means for estimation have been desired.